1. Field of the Invention
The present invention relates to an outer joint part used as an outer member of a ball joint, and a preform formed into an outer joint part.
2. Description of the Related Art One kind of such an outer joint part is disclosed in Japanese Utility Model Application First Publication No. 63-174939. The outer joint part includes a cup-shaped body portion having a plurality of grooves on its inner circumferential surface. The outer joint part is made from a preform by plastic working using outer and inner dies coaxially arranged. The preform includes a bowl-shaped portion having recesses on its inner circumferential surface. The inner die is formed with a plurality of projections on its outer peripheral surface and adapted to force the preform against the outer die such that the projections press onto the recesses of the preform. The grooves of the outer joint part are thus formed corresponding to the recesses of the preform. The preform has an outer circumferential surface of a generally circular shape in cross section.
In the preform of the conventionally proposed type described above, a preform portion formed with the recesses has a reduced thickness in a cross section taken in a radial direction of the preform and the remaining preform portion has an increased thickness in the cross section. A difference in thickness between the preform portion with the recesses and the remaining preform portion causes a difference in rigidity therebetween. The remaining preform portion is not readily deformed in plastic working as compared with the preform portion with the recesses, due to the increased rigidity greater than that of the recessed preform portion. It is likely that the preform of the conventionally proposed fails to be formed into the outer joint part having a predetermined shape on its inner circumferential surface.
Particularly, It is difficult to form the conventionally proposed preform into the outer joint part defining a generally spherical cavity and having a so-called undercut shape in which an inner diameter of the outer joint part is considerably smaller at its open end portion than at its middle bulging portion. In this case, it will be required that the finished cage is subject to machining on the inner circumferential surface in order to increase the inner diameter at the middle portion.
It is an object of the present invention to provide an outer joint part capable of being easily formed.
It is a further object of the present invention to provide a preform readily formed into the cage.